The present invention relates generally to a method for enhancing and evaluating seismic data.
In seismic prospecting, it is conventional to place a plurality of seismic receivers along the earth's surface at spaced locations. A plurality of seismic sources disposed at spaced locations along the earth's surface can then be activated to generate seismic waves which propagate outwardly in all directions. Vibrating devices, explosive devices, and impulsive devices are exemplary of such seismic sources. The seismic waves thus generated are reflected, refracted, and diffracted from subsurface formation interfaces and some of these diverted seismic waves are detected by the seismic receivers and can be processed to form a seismic signal. Such seismic signals can be displayed as seismic sections which contain information about the time, duration and intensity of the diverted seismic waves. The seismic sections can be studied to extrapolate information regarding the type and location of subsurface formations producing the diverted seismic waves. This information, in turn, can be employed to evaluate the subsurface formations for oil- and gas-bearing properties.
Because of the geometry involved, seismic waves reflected from a common reflection point can be received by a first seismic receiver from seismic energy generated by a first seismic source and also by a second seismic receiver from seismic energy generated by a second seismic source. These phenomena are employed in developing common depth point (CDP) seismic data. Each source and receiver pair generating a seismic signal having a common reflection point has a unique range or offset (i.e., the distance separating each source and receiver pair is unique). In practice, seismic data are collected and sorted so that the seismic signals from 3-48 source and receiver pairs having a common reflection point form a "CDP gather" of seismic signals.
The CDP method thus obtains multiple seismic signals for each common reflection point. Because of this redundancy, the seismic signals of a CDP gather can be summed to increase signal-to-noise ratio by reinforcing coherent events within the seismic signals and suppressing random noise. Prior to summing or stacking the seismic signals of a CDP gather, the seismic signals are first processed using normal moveout correction techniques to compensate for the different ray paths of the propagating seismic energy to and from a common reflection point. Summing CDP gathers of moveout corrected seismic signals can thus enhance primary reflection events in the seismic signals which correspond to the propagation of the seismic energy along an assumed ray path to a reflecting interface and at an assumed velocity while suppressing random noise as well as certain multiple reflections of the seismic energy from the reflecting interface.
Once seismic signals of a CDP gather are summed so as to enhance primary reflection events and suppress random noise and multiple reflection events, it is not possible to retrieve the individual seismic signals in such CDP gather for further processing. Thus, processing techniques as described by Ostrander in U.S. Pat. Nos. 4,316,267 and 4,316,268 and by Bodine, et. al., in U.S. Pat. No. 4,646,239 which identify and evaluate amplitude variations as a function of range for selected reflection events in CDP gathers of seismic signals are deprived of a powerful tool for reducing random noise and multiple reflection events while enhancing primary reflection events.
The techniques described by Ostrander and Bodine, et. al., are commonly referred to as "bright spot" analysis or "range dependent amplitude" (RDA) analysis. In fact, RDA analysis techniques have proven to be a powerful exploration tool through their ability to identify seismic reflection events which are likely to correspond to hydrocarbon-bearing formations. RDA analysis techniques have greatly improved exploration success and thus, the importance of utilizing RDA analysis techniques are self-evident. In spite of the potential success to be achieved using RDA analysis techniques, the actual use of RDA analysis techniques is oftentimes unsuccessful because of multiple reflection events and random noise in the seismic data.
Prior to applying RDA analysis techniques, gain correction techniques have generally been used to amplify the seismic signals so as to compensate for temporal and spatial variations in recorded seismic signal amplitude. However, when gain correction techniques have been applied to seismic data having significant random noise and/or multiple reflection events, such gain correction techniques can also have the undesirable effect of modifying geologically-induced amplitude variations (as a function of range) in the seismic data. Consequently, RDA analysis techniques can be ineffective on seismic data having significant random noise and/or multiple reflection events.
The present invention provides a novel method for suppressing random noise and coherent noise (including multiple reflection events) and for enhancing primary reflection events while preserving geologically-induced amplitude variations as a function of range for the primary reflection events. By suppressing random noise and coherent noise while preserving geologically-induced amplitude variations as a function of range for primary reflection events, one can substantially increase the success of employing RDA analysis techniques, whereby attributes descriptive of amplitude variations as a function of range can be obtained for selected primary reflection events. Such attributes can be used to evaluate primary reflection events for their correlation with hydrocarbon-bearing formations.